SnowflakeDB - Add username customization

[MilenaHC]

Overview

Adds the ability to customize username generation for dynamic users in SnowflakeDB.

Uses the new field username_template with the go template language.

[MilenaHC]

Test

Ran existing acceptance tests and manually tested that username customization works as expected

=== RUN   TestSnowflakeSQL_Initialize
--- PASS: TestSnowflakeSQL_Initialize (1.46s)
=== RUN   TestSnowflake_NewUser
=== RUN   TestSnowflake_NewUser/name_creation
=== RUN   TestSnowflake_NewUser/username_creation
=== RUN   TestSnowflake_NewUser/empty_creation
2021/07/01 15:13:24 query issue: 001003 (42000): SQL compilation error:
syntax error line 1 at position 10 unexpected '<EOF>'.
--- PASS: TestSnowflake_NewUser (7.44s)
    --- PASS: TestSnowflake_NewUser/name_creation (3.45s)
    --- PASS: TestSnowflake_NewUser/username_creation (3.05s)
    --- PASS: TestSnowflake_NewUser/empty_creation (0.94s)
=== RUN   TestSnowflake_RenewUser
--- PASS: TestSnowflake_RenewUser (6.23s)
=== RUN   TestSnowflake_RevokeUser
=== RUN   TestSnowflake_RevokeUser/default_revoke
=== RUN   TestSnowflake_RevokeUser/name_revoke
=== RUN   TestSnowflake_RevokeUser/username_revoke
--- PASS: TestSnowflake_RevokeUser (9.97s)
    --- PASS: TestSnowflake_RevokeUser/default_revoke (3.50s)
    --- PASS: TestSnowflake_RevokeUser/name_revoke (2.90s)
    --- PASS: TestSnowflake_RevokeUser/username_revoke (3.57s)
=== RUN   TestSnowflakeSQL_DefaultUsernameTemplate
--- PASS: TestSnowflakeSQL_DefaultUsernameTemplate (2.63s)
=== RUN   TestSnowflakeSQL_CustomUsernameTemplate
--- PASS: TestSnowflakeSQL_CustomUsernameTemplate (2.52s)
PASS
ok      github.com/hashicorp/vault-plugin-database-snowflake    30.416s