Add Base64.IsValid and allow Base64.DecodeXx methods to skip whitespace

[stephentoub]

Cherrypicks, addresses merge conflicts, and squashes the commits from @heathbm in #79334, then adds an additional commit to clean up a few things in the src.

Fixes#76020
cc: @gfoidl, @MihaZupan, @tarekgh

@bartonjs, I looked at PemEncoding.TryFind, which is what drove us to add the decodingLength outs to IsValid, but it seems that implementation also needs to be able to find a base64-encoded value within the region rather than validating the whole thing?

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Issue Details

---

Cherrypicks, addresses merge conflicts, and squashes the commits from @heathbm in #79334, then adds an additional commit to clean up a few things in the src.

Fixes #76020
cc: @gfoidl, @MihaZupan, @tarekgh

@bartonjs, I looked at PemEncoding.TryFind, which is what drove us to add the decodingLength outs to IsValid, but it seems that implementation also needs to be able to find a base64-encoded value within the region rather than validating the whole thing?

Author:	stephentoub
Assignees:	-
Labels:	area-System.Memory, breaking-change, new-api-needs-documentation, needs-breaking-change-doc-created
Milestone:	-

[stephentoub]

Breaking change doc issue:
dotnet/docs#35282

[stephentoub]

Failures are #85949

[stephentoub]

I looked at PemEncoding.TryFind, which is what drove us to add the decodingLength outs to IsValid, but it seems that implementation also needs to be able to find a base64-encoded value within the region rather than validating the whole thing?

On closer look, seems it's just trimming whitespace in a somewhat convoluted manner. I'll submit a PR to replace it with this.