Add a new setting called debug_shell_startup

[JeanChristopheMorinPerso]

Add a new setting called debug_shell_startup. When enabled, it logs information about the shell startup sequence.

This is kind of related to #2058. While helping on that issue, I noticed that it was hard the shell startup code was annoying to debug and more importantly, that part of rez doesn't have a lot of visibility. We don't visit that part very often. I thought that having some logs could help us and others better understand the general setup.

Here is how it looks on my machine:

$ REZ_DEBUG_SHELL_STARTUP=1 rez-env python
12:52:41 DEBUG    Writing context to /tmp/rez_context_cnmb7aaw/context.sh
12:52:41 DEBUG    Writing shell script to /tmp/rez_context_cnmb7aaw/.profile
12:52:41 DEBUG    Writing shell script to /tmp/rez_context_cnmb7aaw/.bash_profile
12:52:41 DEBUG    Writing shell script to /tmp/rez_context_cnmb7aaw/.bashrc
12:52:41 DEBUG    Setting $HOME for new shell to /tmp/rez_context_cnmb7aaw
12:52:41 DEBUG    Writing rez-shell to /tmp/rez_context_cnmb7aaw/rez-shell.sh
12:52:41 DEBUG    Launching shell with command: /usr/bin/bash /tmp/rez_context_cnmb7aaw/rez-shell.sh

You are now in a rez-configured environment.
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[codecov]

Codecov Report

❌ Patch coverage is 17.39130% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.94%. Comparing base (a0d2ddb) to head (a461bdc).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/shells.py	11.11%	6 Missing and 2 partials ⚠️
src/rezplugins/shell/_utils/powershell_base.py	20.00%	2 Missing and 2 partials ⚠️
src/rezplugins/shell/cmd.py	20.00%	2 Missing and 2 partials ⚠️
src/rez/resolved_context.py	33.33%	1 Missing and 1 partial ⚠️
src/rez/rezconfig.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2059      +/-   ##
==========================================
- Coverage   59.99%   59.94%   -0.05%     
==========================================
  Files         163      163              
  Lines       20118    20139      +21     
  Branches     3519     3528       +9     
==========================================
+ Hits        12070    12073       +3     
- Misses       7227     7238      +11     
- Partials      821      828       +7     

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