providers: add opt-out build tags to reduce binary size

[ljuti]

Add opt-out build tags so users can exclude providers they don't use, reducing binary size significantly.

All providers compile by default — no breaking change. Pass -tags to exclude heavy dependency trees:

go build -tags "nogoogle,noaws,noazure,novercel,noopenrouter,nokronk" ./...

Tags

Tag	Excludes
nogoogle	providers/google + anthropic vertex support
noaws	providers/bedrock + anthropic bedrock support
noazure	providers/azure
novercel	providers/vercel
noopenrouter	providers/openrouter
nokronk	providers/kronk

Implementation

• Every .go file in each optional provider package gets //go:build !no<name>
• providertests/ files that import optional providers get matching tags
• Anthropic's bedrock.go / google.go gated behind !noaws / !nogoogle
• Bedrock/Vertex LanguageModel configuration extracted into build-tagged files (bedrock_lm.go, vertex_lm.go) with error-returning stubs for when excluded
• anthropic.go heavy imports (AWS SDK, Google OAuth) removed from main file

Binary size impact

Stripped -ldflags="-s -w" binary with anthropic + openai + openaicompat:

	Size
All providers compiled	32 MB
Core only (all tags set)	12 MB
Savings	20 MB (63%)

[ljuti]

@andreynering Opening this as a draft PR because I'm not sure if this is a direction that you want to go with the library. Feel free to reject and close if not aligned with project goals.

I'm building and maintaining an internal toolset that uses Fantasy and I'd like to keep the dependencies as clean as possible. Currently, my plan is to internally support only anthropic and openai-compat providers, and that is the reasoning behind this proposed modification. I'm happy to maintain my own fork with these modifications if this will not be accepted to the project.