Move general Open5GS OpenAPI scripts, man pages and templates to a new top level directory

[davidjwbbc]

This moves the generate-openapi script and man page from the 5G Media Streaming specific directory to a general use open5gs-tools directory. The Mustache templates for the C language, which are currently used by the 5GMS Application Function, are also moved to this general area.

This move does not include any current overrides for the 3GPP 5G_APIs files, currently used by the 5GMS build systems, as these will be placed on specific Release 17 and Release 18 branches to track changes on the equivalent branches in the 3GPP 5G_APIs repository.

5GMS Specific override/supplementary OpenAPI YAML files, that are used by 2 or more 5GMS reference tools repositories, will remain where they are.

The "Maf_Management" OpenAPI YAML, which is specific to the reference tool 5GMS Application Function, will be moved into the rt-5gms-application-function repository in a separate PR.

This forms part of the rt-common-shared part of the fix for issue #33. To complete:

• Create Rel17 & Rel18 branches (already done - new branches cannot be PR'd)
• Use new branches in 5GMS Application Function and 5GC Data Collection AF repositories (PRs on the relevant repository).

[davidjwbbc]

Slight change of plan:

• TS 26.532 YAML overrides will move into data-reporting/5G_APIs-overrides on the 3GPP-Rel17 & 3GPP-Rel18 branches with an empty directory on main (README file in the overrides directory to explain why it's empty in main branch).
• TS 26.512 YAML override to move to a src/5gmsaf/5G_APIs-overrides directory in the rt-5gms-application-function repository.
• open5gs-tools/5G_APIs-overrides will be removed