Get faucet url from envvar or resolve srv record

[pnowosie]

SRV record is set (thanks to @Wiezzel who figured it out).

 nslookup -type=SRV _zk-faucet._tcp.dev.golem.network
Server:		127.0.0.53
Address:	127.0.0.53#53

Non-authoritative answer:
_zk-faucet._tcp.dev.golem.network	service = 10 10 5778 yacn.dev.golem.network.

Authoritative answers can be found from:

Testing shows that SRV record is resolved only to hostname & port
2021-02-03T07:29:38Z INFO faucet_srv] Resolved faucet host: "yacn.dev.golem.network:5778"

Assumptions

For now I'm taking the following assumptions (tech debt)

• all other zk-sync faucets we provide via SRV would end with zk/donatex
• environment variable ZKSYNC_FAUCET_ADDR value is taken as is without any changes

Tests

Run withdrawal example with log level debug. Example of [successful withdrawal].

[2021-02-03T08:11:17Z DEBUG ya_utils_networking::srv_resolver] Resolved address: yacn.dev.golem.network:5778
[2021-02-03T08:11:17Z DEBUG ya_zksync_driver::zksync::faucet] Faucet url: http://yacn.dev.golem.network:5778/zk/donatex/0x19a2...6df3

[Wiezzel]

Reviewed & tested. OK ☑️
Please change PR base to payments/beta-1