enhancement: Easier configuration of the postgres command

[KyleAure]

Currently, the PostgreSQLContainer configures fync=off during initialization of the container class.
This setting can be accidentally overwritten by using the withCommand() or setCommand() methods.
It would be more extensible to keep a list of configuration options for the postgres command, and instead configure a command during the configure lifecycle step.

Opening this PR because this is how I use the PostgreSQL container and thought maybe others might find this useful.
If there is no interest feel free to close this PR.

[richardfearn]

Slightly related to #938

[richardfearn]

I would also love a way to do this via the JDBC URL. The 'best' option I've come up with so far is a new TC_ (container) parameter for a list of comma-separated, URL-encoded, additional command-line options to add to the command. The configure method in PostgreSQLContainer can then split/decode them and append them to the command.

The URL could look something like this, for example:

jdbc:tc:postgresql:15.10:///integration_tests?TC_INITSCRIPT=initPostgres.sql&TC_COMMAND_EXTRA=-c,log_statement%3Dall

[KyleAure]

@richardfearn Thanks for posting an alternative option for others.
Unfortunately, for many of my complex test cases I cannot use the Testcontainer based JDBC URL.