Add MCP Server Card (SEP-2127) types + handler (#2768)

Define the GitHub MCP Server's Server Card (SEP-2127, in review) and a
public no-auth HTTP handler that serves it at the canonical /server-card
backend path. OSS owns all stable identity/metadata and serving behavior;
the remote server only supplies an environment-specific remote URL.

Negotiate Accept and If-None-Match as RFC 9110 list values across repeated
header field-lines. Accept honors quoted commas, media-range parameters, and
q-values (q=0 rejects); entity-tag lists split on quoted commas without
backslash escaping, since an opaque-tag treats a backslash literally. Vary
lists Accept and X-Forwarded-Host and is appended rather than set: it
composes with values added by deployment middleware and, since the response
is publicly cacheable and multi-tenant deployments derive the remote URL from
the trusted X-Forwarded-Host, it keeps a shared cache from serving one
tenant's card to another.

Refs github/copilot-mcp-core#1855, epic github/copilot-mcp-core#1853
Spec: modelcontextprotocol/experimental-ext-server-card

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 4a9f522f-6942-4b77-98a4-b2d42f19625d