Build time default network selection and custom network support

[urtho]

This PR adds support for a build time custom network definition (name, id, endpoints, feesink).
It also adds (optional) default network selection.

[PatrickDinh]

Hi @urtho, thanks for the PR. From my understanding, you are adding 2 features into lora:

• Allow a custom network at build time. Optionally, a default selected network can be configured
• Allow a fee sink address to be specified for the custom network

May I ask why do you need the "custom network at build time" feature? Currently, lora supports custom network already. If you head to Lora settings, you can create one

Additionally, I think supporting custom network at build time isn't flexible because:

• There only 1 can be configure
• If the config needs to be changed, the entire lora would need to be rebuild

I think the second feature "fee sink address for custom network" is very valuable. In the transaction wizard, currently we auto populate the sender address with the fee sink address, but only for testnet, fnet, betanet and localnet. Therefore, allowing this for custom network would give us a lot of flexibility. Do you want to update the existing Lora network settings feature to add fee sink address to custom network?

[urtho]

Hi, thanks for taking a look at this PR.

This is intended for private AVM networks that I run (think Algorand L2s ;) )
Users expect the dedicated Lora instance to just work without having to configure anything.

My wording is also incorrect, as the PR is not about build-time support.
It adds support for custom network prefill using ENV variables.