grpc: add basic htpasswd authentication support

[mostynb]

Add basic htpasswd auth to the gRPC endpoint, for feature parity with the HTTP endpoint. Until now, gRPC has been unauthenticated.

To test, add the user/pass to bazel like so:
--remote_cache=grpc://user:pass@address:port/

[abcdefstar]

Hi @mostynb,
I tried passing the user:pass from htpasswd file to the bazelrc file, -grpcs://user:pass@host:port --remote_header=x=test. But I get an error as ERROR: Failed to query remote execution capabilities: UNIMPLEMENTED: HTTP status code 404. Do we need to store these credentials as a secret in kubernetes ? I have added this as part of my deployment -

• --htpasswd_file=/pathtomyfile
• --allow_unauthenticated_reads

[mostynb]

Let's follow up in #575.