Make use of public_key:cacerts* functions

[lukebakken]

Is your feature request related to a problem? Please describe.

Erlang/OTP 25 introduced a new API to fetch system CA certificates:
https://www.erlang.org/blog/my-otp-25-highlights/#ca-certificates-can-be-fetched-from-the-os-standard-place

...and, httpc will use this API to fetch CA certs if no specific certs are specified.

We should investigate if / how to integrate this into RabbitMQ's various TLS settings and usage of ranch, ssl, httpc, TLS-encrypted disterl, etc

Describe the solution you'd like

Figure out a "least surprises" way to use the OS CA certificates.

Describe alternatives you've considered

N/A

Additional context

N/A