Support DEX orders on stellar client

[AhmedHanafy725]

Which package/s are you suggesting this feature for?

stellar_client

Is your feature request related to a problem? Please describe

threefoldtech/threefold_connect#775

Describe the solution you'd like

support placing, canceling, and listing orders

[AlaaElattar]

Work in progress

• all done except that submitting any order not working with threefold service so still debugging in it.

[AlaaElattar]

Blocked

• couldn't get any info about how to use TF service with creating/cancelling orders, only supported that user should have xlms to make any orders.

[AlaaElattar]

Work In Progress:

• Adding docs to each function and its requirements for more explanation.
• Fixing pr comments

[AlaaElattar]

Work Completed:

• Added documentation for functions explaining all details.
• Added _getAsset to remove duplicated code across functions.

[AlaaElattar]

Work Completed:

• Updated docs about stellar representation of orders.