internal/transport: validate balancer and address metadata (#9203)

Fixes #9199

Validate metadata supplied by balancer `PickResult.Metadata` before
merging it into the outgoing context, and validate resolver/address
metadata before converting it to HTTP/2 header fields.

The tests cover invalid address metadata at the transport boundary and
invalid balancer metadata through an end-to-end RPC path.

RELEASE NOTES:
* transport: Invalid metadata supplied by balancers or resolver
addresses is rejected before request headers are created.