Allow show techsupport to save with a custom filename

[donggyu-nexthop]

What I did

fixes #4503
added the ability to specify custom techsupport dump filenames

How I did it

• Added support for an optional -f/--filename flag for show techsupport.
• updated generate_dump script to take in the same flag

In the event of filename collisions, generate_dump will log the warning to stdout, and exit with code 1 without overwriting.

How to verify it

on a DUT:

show techsupport -f testdump

# verify the dump appeared
ls -l /var/dump | grep testdump

Previous command output (if the output of a command-line utility has changed)

The --filename flag wasn't supported before this PR, so the command would error out.

New command output (if the output of a command-line utility has changed)

$ show techsupport -f testdump
Lock successfully acquired and installed signal handlers
mkdir: created directory '/var/dump/testdump'
'/var/dump/testdump/generate_dump' -> '/usr/local/bin/generate_dump'

...

Cleaning up working directory /var/dump/testdump
Removing lock. Exit: 0
removed '/tmp/techsupport-lock/PID'
removed directory '/tmp/techsupport-lock'
/var/dump/testdump.tar.gz

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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