fix: support headers as metadata with gRPC + reflection

[ryan-willis]

resolves: INS-4850

• always show the Headers tab in the gRPC request pane in order to send custom headers with the reflection requests (closes Cannot specify headers for gRPC server reflection #6865)
• combines the headers from the ancestors into the gRPC metadata (also applies to reflection)
• in addition to filtering out disabled headers from gRPC metadata, removes ones that would throw errors (header keys with an empty name)

Opted to keep the "gathering" code localized to the pane since we're doing a whole lot in there already, but I'd like to move a good portion of the code in here into the loader (if feasible) in the future.

Intentionally did not surface validation errors (empty metadata/header keys) in the editor because (I believe) it'll be apparent enough to users visually that empty headers will not be sent