Support for custom api version for about endpoint

[tanghel]

Reasoning

• External api providers maybe don't have git installed or start the service from a private repository where the application / plugins commit hash is not relevant, in which case a custom version can be read from the environment variables

Proposed Changes

• support for returning api version from a custom environment variable
• if the git commands somehow fail, still return the other relevant information

How to test

• when starting the service with API_VERSION environment variable, it should be reflected in the /about endpoint

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 2.94118% with 33 lines in your changes are missing coverage. Please review.

Project coverage is 46.19%. Comparing base (41bd507) to head (2e333e5).

Files	Patch %	Lines
src/endpoints/network/network.service.ts	0.00%	29 Missing and 4 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1236 +/- ## =============================================== - Coverage 46.26% 46.19% -0.08% =============================================== Files 594 594 Lines 15826 15850 +24 Branches 1695 1696 +1 =============================================== Hits 7322 7322 - Misses 6874 6897 +23 - Partials 1630 1631 +1 ``` | [Flag](https://app.codecov.io/gh/multiversx/mx-api-service/pull/1236/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=multiversx) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/multiversx/mx-api-service/pull/1236/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=multiversx) | `46.19% <2.94%> (-0.08%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=multiversx#carryforward-flags-in-the-pull-request-comment) to find out more.

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