Support for Polkadot v0.8.28

[pablojorge]

This PR adds support for two changes introduced in v0.8.28 (https://github.com/paritytech/polkadot/releases/tag/v0.8.28):

• New MultiAddress enum (More Extensible Multiaddress Format paritytech/substrate#7380, Use Extensible Multiaddress in all Polkadot Runtimes paritytech/polkadot#2126)
• New 'providers' reference counter in the account store (Introduces account existence providers reference counting paritytech/substrate#7363, Companion for #7363 (Account-providers ref-counting) paritytech/polkadot#2152)

A new MultiAddress class that takes the place and wraps the original Address is introduced. The BalanceTransfer and Extrinsic classes now contain a reference to a MultiAddress, and for backward compatibility, we keep setters that accept a plain Address instance.

[codecov-io]

Codecov Report

Merging #50 (5d09ba6) into master (b2d8775) will increase coverage by 0.13%.
The diff coverage is 94.44%.

@@             Coverage Diff              @@
##             master      #50      +/-   ##
============================================
+ Coverage     86.91%   87.04%   +0.13%     
- Complexity      892      911      +19     
============================================
  Files           115      118       +3     
  Lines          2973     3034      +61     
  Branches        430      438       +8     
============================================
+ Hits           2584     2641      +57     
  Misses          278      278              
- Partials        111      115       +4     

Impacted Files	Coverage Δ	Complexity Δ
.../io/emeraldpay/polkaj/scaletypes/MultiAddress.java	87.50% <87.50%> (ø)	2.00 <2.00> (?)
...in/java/io/emeraldpay/polkaj/scale/UnionValue.java	94.44% <87.50%> (-5.56%)	12.00 <7.00> (+7.00)	⬇️
...a/io/emeraldpay/polkaj/scaletypes/AccountInfo.java	100.00% <100.00%> (ø)	17.00 <5.00> (+3.00)
...meraldpay/polkaj/scaletypes/AccountInfoReader.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
.../emeraldpay/polkaj/scaletypes/BalanceTransfer.java	74.07% <100.00%> (+2.07%)	15.00 <1.00> (+1.00)
...ldpay/polkaj/scaletypes/BalanceTransferReader.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
...ldpay/polkaj/scaletypes/BalanceTransferWriter.java	100.00% <100.00%> (ø)	3.00 <1.00> (+1.00)
...ava/io/emeraldpay/polkaj/scaletypes/Extrinsic.java	93.75% <100.00%> (+0.20%)	11.00 <0.00> (ø)
.../emeraldpay/polkaj/scaletypes/ExtrinsicReader.java	84.37% <100.00%> (ø)	2.00 <0.00> (ø)
.../emeraldpay/polkaj/scaletypes/ExtrinsicWriter.java	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
... and 5 more

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[splix]

@pablojorge sorry, totally missed the Pull Request. Just checked it, everything looks good, and I'm going to merge it now. Thank you!