Add existence requirement

[zjb0807]

Add ExistenceRequirement parameter to transfer/withdraw of MultiCurrency and BasicCurrency traits

[codecov]

Codecov Report

Attention: Patch coverage is 59.57447% with 19 lines in your changes missing coverage. Please review.

Project coverage is 65.40%. Comparing base (286c731) to head (25f5a0a).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
currencies/src/lib.rs	53.57%	13 Missing ⚠️
xcm-support/src/currency_adapter.rs	37.50%	5 Missing ⚠️
payments/src/lib.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1014      +/-   ##
==========================================
- Coverage   65.63%   65.40%   -0.23%     
==========================================
  Files          49       49              
  Lines        3704     3726      +22     
==========================================
+ Hits         2431     2437       +6     
- Misses       1273     1289      +16     

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