Enable way to specify static values for custom HTTP headers

[CodingDoug]

sqld instances might have an interest in exposing some of their configuration to client apps in the form of HTTP headers, especially for the purpose of logging and diagnostics. One way to do this is read environment variables, perhaps all with the same prefix, as the names and values to send. For example, a sqld might want to expose its location to client, so setting LIBSQL_SQLD_LOCATION=foo in the env could result in sending the header X-Sqld-Location: foo in every response.

cc @athoscouto