fix(NODE-3675): SRV option bug correctly defaults authSource to $external

[syz99]

Description

What is changing?

Previously, for external mechanisms, it was not always the case that authSource defaulted to $external even though that was the intended behavior / what the spec outlines. External auth source mechanisms are now explicitly checked for in connection_string to avoid errant overrides (e.g. authSource = admin over $external). Unit tests added as well.

Is there new documentation needed for these changes?

n/a

[ jira ]

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: <type>(NODE-xxxx)<!>: <description>
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[syz99]

I've chosen to replace any checks for external auth mechanisms (e.g. if (GSSAP || AWS || X509)) with a check for the newly introduced $EXTERNAL_AUTH_SOURCE_MECHANISMS set "typing".

This is mainly a choice of grouping these three as the external auth source mechanisms explicitly. Does this introduce too much confusion / make things less readable?

example diffs: (1), (2)

[nbbeeken]

LGTM, thanks!