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?

[ghost]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-memory
See info in area-owners.md if you want to be subscribed.

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.