feat: Add x-goog-api-client header to rest clients

[vam-google]

Also add Content-Type: application/json header.

This PR depends on googleapis/python-api-core#189, and assumes that google-api-core version 1.27.0 has been already released (which was not the case on the moment of creation of this PR).

[codecov]

Codecov Report

Merging #888 (870124f) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #888 +/- ##
=========================================
Coverage 100.00% 100.00% =========================================
Files 26 27 +1 Lines 1608 1697 +89 Branches 328 347 +19 =========================================
+ Hits 1608 1697 +89 

Impacted Files	Coverage Δ
gapic/samplegen_utils/types.py	100.00% <ø> (ø)
gapic/samplegen_utils/utils.py	100.00% <ø> (ø)
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/samplegen/samplegen.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/metadata.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
... and 3 more

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[software-dov]

Naive question: this is not x-goog-api-client. Where does the name content-type come from? Is there documentation somewhere describing it?

[vam-google]

The Content-Type header is a part of HTTP standard: https://datatracker.ietf.org/doc/html/rfc2616#section-14.17

[vchudnov-g]

@software-dov See also the internal REGAPIC gRPC transcoding design doc, where we call this out explicitly (relatively recent addition, since we found some RPCs fail without it)

[vam-google]

@busunkim96 PTAL

[busunkim96]

LGTM.

I suspect the coverage drop is from the clauses I added to keep compatibility with older google-auth and google-api-core versions in 89d6f35#diff-f7a16a65f061822bcc73b8296f4dc837353d379d8d9cc5307982cb6941442835.

[busunkim96]

Opened PR to remove code / tests for google-api-core < 1.26.0. See #893

Once that is merged updating this branch should resolve the coverage issues.

[busunkim96]

@vam-google CI is now passing.