Extract inventory build logic to a standalone function

[AlexBasinov]

Extract the inventory file creation logic from the main install-oracle.sh script into a standalone build_inventory_file() function. This function now resides in its own standalone file to allow for potential reuse by other shell scripts. The original Bash code was copied with slight refactoring to improve readability and make the control flow easier to understand.

Internal bug: b/416300768

Testing:

• Single instance: https://gist.github.com/AlexBasinov/1fc2fcd8dbdbcbb94514d8fce55e363a
• RAC: https://gist.github.com/AlexBasinov/14da80f5c4d2bbe914e53fee025a047b

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AlexBasinov

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[google-oss-prow]

@AlexBasinov: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
oracle-toolkit-install-single-instance-on-bms	f550802	link	true	/test oracle-toolkit-install-single-instance-on-bms
oracle-toolkit-install-rac-on-bms	f550802	link	true	/test oracle-toolkit-install-rac-on-bms

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