Providers config and custom providers

[guidograzioli]

Configuring providers

Variable	Description	Default
keycloak_quarkus_providers	List of provider definitions; see below	[]

Provider definition:

keycloak_quarkus_providers:
  - id: http-client                         # required
    spi: connections                        # required if url is not specified
    default: true                           # optional, whether to set default for spi, default false
    restart: true                           # optional, whether to restart, default true
    url: https://.../.../custom_spi.jar     # optional, url for download
    properties:                             # optional, list of key-values
      - key: default-connection-pool-size
        value: 10

the definition above will generate the same configuration as the following build command:

bin/kc.sh build --spi-connections-provider=http-client --spi-connections-http-client-default-connection-pool-size=10

Fix #131

[rpelisse]

LGTM