feat: add BIGNUMERIC support

[plamut]

Closes#367.

This is a continuation of #447 that adds extra tests and a few final touches.

PR checklist:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[plamut]

@HemangChothani If you see this, can you please confirm to the bot that you consent using your commits? (this PR builds on top of your work)

Thanks!

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[plamut]

Tests will have to be refactored to not assume pyarrow>=3.0.0 is available (they fail when they are run with pyarrow==1.0.0) - the BIGNUMERIC fixtures and assertions need to be moved out and skipped conditionally.

[tswast]

Looking good! One suggestion regarding the version comparison. We also will need some skipifs. Might be able to do those based on the pattern we have in google-auth, with a private constant that indicates if the BIGNUMERIC feature is available.

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[tswast]

One question, but otherwise looks good

[plamut]

@shollyman Can you please take a look at the CLA? The PR is a continuation of #447 which was opened by an existing maintainer, but I do see that the CLA check has failed there even though the PR was submitted before the before the new year.

(to date the original author has not yet responded here)

[tswast]

"CLAs are signed, but unable to verify author consent"

Since these commits were pulled from #447, I'm going to call that author consent and mark the label as true.