Add NSX-T Firewall support

[Didainius]

This PR adds support for NSX-T Firewall by adding new functions and types:
NsxtEdgeGateway.UpdateNsxtFirewall(), NsxtEdgeGateway.GetNsxtFirewall(), nsxtFirewall.DeleteAll(),
nsxtFirewall.DeleteById, types.NsxtFirewallRule, types.NsxtFirewallRuleContainer

It also fixes a few errors found by new staticheck:

## staticcheck 2021.1 (v0.2.0)

## Checking govcd
api.go:285:61: the argument's underlying type is a slice of bytes, should use a simple conversion instead of fmt.Sprintf (S1025)
api_vcd_test.go:762:3: this value of err is never used (SA4006)
entity_test.go:145:13: type assertion to the same type: ge already has type genericEntity (S1040)
entity_test.go:165:13: type assertion to the same type: ge already has type genericEntity (S1040)
entity_test.go:178:13: type assertion to the same type: ge already has type genericEntity (S1040)
entity_test.go:192:13: type assertion to the same type: ge already has type genericEntity (S1040)
filter_engine.go:253:25: type assertion to the same type: candidate already has type QueryItem (S1040)
filter_engine.go:284:27: type assertion to the same type: candidate already has type QueryItem (S1040)

[lvirbalas]

Nice work! Only one question unrelated to this PR.

[lvirbalas]

Though unlrelated to this PR, I just noticed this line. Do we break the semver aspect of the govcd versioning here?

CC: @dataclouder

[Didainius]

Yep - this happened as part of #372