[AURON #2460] Add --mvn, --goal and -- passthrough to auron-build.sh

[ShreyeshArangath]

What changes were proposed in this pull request?

Fixes #2460.

Three small additions that make auron-build.sh usable as a build step inside another script, and on machines without network access:

• --mvn <PATH> builds with an already installed Maven instead of build/mvn, which downloads Maven and so fails in an offline or airgapped build. It accepts either a path or a command on PATH, and errors if it resolves to neither.
• --goal <GOAL> sets the Maven goal. It defaults to install, so nothing changes for existing callers, but a caller that only wants artifacts can run package and leave the local repository alone.
• -- ends option parsing, so everything after it reaches Maven unchanged. auron-build.sh fails to parse options after -D arguments #2227 fixed -D handling via MVN_D_ARGS, but -P, -B and long options such as --no-transfer-progress are still either swallowed by the break in the -* handler or rejected as an unknown option. The -- arm sits alongside the existing -D handling rather than replacing it, and is placed before the --* arm so it is not caught as an unknown option.

This is independent of #2461 and the two can merge in either order.

How was this patch tested?

Manually, with a stub Maven that echoes its arguments so the composed command line is visible.

All three together:

$ bash auron-build.sh --release --mvn /tmp/stubmvn --goal package \
    --sparkver 3.1 --scalaver 2.12 -- -Pceleborn-0.6 -B --no-transfer-progress
STUB_MVN_INVOKED_WITH: clean package -DskipTests -Prelease -Pspark-3.1 -Pscala-2.12 -Pceleborn-0.6 -B --no-transfer-progress

The goal is package rather than install, and -P, -B and the long option all arrive intact.

Also checked:

• Without --goal the command line still says clean install, so existing callers are unaffected.
• -DskipBuildNative placed among the options still lands in MVN_D_ARGS as before, so auron-build.sh fails to parse options after -D arguments #2227 behaviour is preserved.
• --mvn resolves a bare command name off PATH as well as an explicit path.
• --mvn /nope/mvn exits 1 with ERROR: --mvn '/nope/mvn' is not an executable or a command on PATH.
• --goal with no argument exits 1 with a message naming the expected values.
• An unknown long option such as --bogus is still rejected, so the new -- arm does not weaken that check.