feat: Refactor fstab parsing and add plugins for /etc/fstab and /proc/mounts

[Yld2004]

Description

Summary

This PR extracts the logic for parsing fstab-style entries into a reusable utility function and introduces two new plugins: FstabPlugin and MountPlugin. These changes allow for consistent extraction of mount information from both static configuration files and the volatile /proc environment.

Key Changes

Logic Centralization:

Created parse_fstab_entry in dissect/target/plugins/os/unix/_os.py.

Updated parse_fstab to use this new centralized helper, improving handling of whitespace, comments, and field padding (defaults).

New Plugins:

FstabPlugin: Located at etc.fstab, it yields records for entries found in /etc/fstab.

MountPlugin: Located at linux.proc.mounts, it iterates through all processes in /proc and yields their respective mount points in the /proc/<pid>/mounts file.

Process Model Enhancement:

Added a mounts() method to the Process class in the Linux proc plugin, allowing per-process mount inspection.

Introduced FstabEntry dataclass for structured data handling within the proc module.

Testing:

Added comprehensive tests for the new plugins (test_fstab.py, test_mounts.py).

Updated conftest.py to include mock mount data for Linux process testing.

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[Yld2004]

@DissectBot agree

[Schamper]

Can you fix the linting errors?

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.14%. Comparing base (f66d3dc) to head (4a25b21).

Files with missing lines	Patch %	Lines
dissect/target/plugins/os/unix/_os.py	91.42%	3 Missing ⚠️
dissect/target/plugins/os/unix/linux/proc.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1613      +/-   ##
==========================================
+ Coverage   81.10%   81.14%   +0.03%     
==========================================
  Files         401      403       +2     
  Lines       35175    35263      +88     
==========================================
+ Hits        28529    28613      +84     
- Misses       6646     6650       +4     

Flag	Coverage Δ
unittests	81.14% <96.00%> (+0.03%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

---

_{Comparing Yld2004:mount-and-fstab (4a25b21) with main (f66d3dc)}

[Schamper]

I meant more something like this:

Suggested change

	for line in fstab_data:
	with fstab.open("rt") as fh:
	for line in fh:

[Yld2004]

I wanted to do it but below it will sum up into 5 tabs Don't you think it will be too much tabs?

[Schamper]

It's fine. We have a 120 width line limit. Plenty of space :)