Feature implementation from commits 28128e0..d2e8366

[yashuatla]

This PR contains changes from a range of commits from the original repository.

Commit Range: 28128e0..d2e8366
Files Changed: 136 (115 programming files)
Programming Ratio: 84.6%

Commits included:

• xds: revert xds: migrate internal xdsclient to use generic client and dedicated LRS client grpc/grpc-go#8310: migration of xdsclient to use generic client and dedicated LRS client
• add spiffe config (test/kokoro: add config for spiffe test grpc/grpc-go#8384)
• xds: migrate internal xdsclient to use generic client and dedicated LRS client (xds: migrate internal xdsclient to use generic client and dedicated LRS client grpc/grpc-go#8310)
• xds: Remove temporary environment variable for least request (xds: remove temporary environment variable for least request grpc/grpc-go#8248)
• cmd/protoc-gen-go-grpc: use Error() since no formatting is performed (cmd/protoc-gen-go-grpc: use Error() since no formatting is performed grpc/grpc-go#8378)
• Add flag guarding SPIFFE Bundle provider (Add flag guarding SPIFFE Bundle provider grpc/grpc-go#8343)
• balancer: Make ExitIdle compulsory for Balancers (balancer: Make ExitIdle compulsory for Balancers grpc/grpc-go#8367)
• deps: update dependencies for all modules and fix revive findings (deps: update dependencies for all modules grpc/grpc-go#8372)
• grpc: introduce new Dial and Server Options to set static window size (grpc: introduce new DialOption and ServerOption to configure initial window size without disabling BDP estimation. grpc/grpc-go#8283)
• xds/internal: update generic grpctransport codec name to proto (xds/internal: update generic grpctransport codec name to proto grpc/grpc-go#8368)
  ... and 5 more commits