Add "--tool <name>" switch for fine grained control over tool list

[joshfree]

Describe the bug

Today we allow users to limit the available MCP tools to a single namespace, for example:

# Start MCP Server with only Storage tools
azmcp server start \
    --namespace storage \
    --mode all \
    [--transport <transport>] \
    [--read-only]

However, for Sub-Agents, some customers want the ability to have even more control; the ability to specify the EXACT one (1) or more tools exposed:

# Start MCP Server with only `postgres database query`
azmcp server start \
    --tool azmcp_postgres_database_query \ 
    --mode all \
    [--transport <transport>] \
    [--read-only]

Expected behavior

Ability to set the exact tool list.

Actual behavior

No ability to control the exact tool list today.

[joshfree]

@Petermarcu