Implement Juniper SRX formatter

[tony-butchart]

Add formatter for Juniper SRX series Firewalls which only support dynamic address lists with IP addresses (host or network) in CIDR notation. I've also updated the Docker readme to include documentation on how to use this formatter.

This can be utilised by any SRX firewall running Junos 18.4R1 or higher as per the following Junos CLI reference document:

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/dynamic-address.html

I have validated this in a lab using my own SRX running Junos 22.3R3-S1.3

user@srx> show security dynamic-address summary

Dynamic-address session scan status            : Disable
Hold-interval for dynamic-address session scan : 10 seconds

  Server Name                 : crowdsec
    Hostname/IP               : http://192.168.1.2:41412
    Update interval           : 30
    Hold   interval           : 86400
    TLS Profile Name          : ---
    User        Name          : ---

    Feed Name                             : crowdsec
        Mapped dynamic address name       : crowdsec-blocklist
        URL                               : http://192.168.1.2:41412/security/blocklist.txt
        Feed update interval              : 30       Feed hold interval :86400
        Total update                      : 16310
        Total IPv4 entries                : 16240

Note that using this formatter successfully does require modifying the endpoint configuration value in cfg.yaml to include a .txt suffix for the SRX to correctly identify the format (SRX also supports multiple .txt address lists packed in the .gz archive format).
I've noted this in the documentation, but this does mean that there would be a limitation in that SRX devices wouldn't support Global RunTime Query Parameters in their current format.

[LaurenceJJones]

Hey 👋🏻

Apologizes I missed this PR, thank you for opening this!

Seems pretty simple, I wont be able to test it on an actual system but if your happy and the documentation is quite sufficient on how users can implement it then LGTM!