por-indexer: Override MAX_PAYLOAD_SIZE_LIMIT to default to 5MB

Default MAX_PAYLOAD_SIZE_LIMIT to 5MB for por-indexer.
- We have seen cases where PoR address lists exceed 5MB, and so cause issues as requests get blocked by fastify and so are not processed by the EA.
- This PR increases the default from 1MB to 5MB, to increase the headroom before we experience these issues.
https://smartcontract-it.atlassian.net/browse/DF-20251

Quality Assurance

[X] If a new adapter was made, or an existing one was modified so that its environment variables have changed, update the relevant infra-k8s configuration file.
[X] If a new adapter was made, or an existing one was modified so that its environment variables have changed, update the relevant adapter-secrets configuration file or update the soak testing blacklist.
[X] If a new adapter was made, or a new endpoint was added, update the test-payload.json file with relevant requests.
[X] The branch naming follows git flow (feature/x, chore/x, release/x, hotfix/x, fix/x) or is created from Jira.
[X] This is related to a maximum of one Jira story or GitHub issue.
[X] Types are safe (avoid TypeScript/TSLint features like any and disable, instead use more specific types).
[X] All code changes have 100% unit and integration test coverage. If testing is not applicable or too difficult to justify doing, the reasoning should be documented explicitly in the PR.