improve null safety

This release improves the null safety functionality of the sdk. Prior to this release, null safety was supported by generated code, with most member variables and parameters of constructors and functions set as optional. We have corrected this, so that it is now precisely defined which variables are not nullable.

Other improvements of this release are:

• performance improvement
• documentation improvement and update
• improvement of test cases

update dependency packages

• update dependency packages to newest versions
• update sdk to work with the newest packages
• fix example app build

Update SEP-005 & SEP-001

• update sep-005 : add malay language
• update sep-001: improve and add new fields

txrep support for protocol 19

adds support for protocol 19 to txrep (SEP-0011)

SEP-0007 Support

adds utility features to facilitate the implementation of SEP-0007 in a Flutter Wallet

Protocol 19 support

Add support for protocol 19, see #42

bugfix stack overflow error

see #44

improvements and bugfixes

• extend example app - add Stellar Quest Series 1 & 2
• bugfixes xdr transaction result parsing - see issue #40

support muxed accounts by default

• Muxed accounts are now supported by default (see #39)
• fix sep-0012 headers
• improve documentation

bugfixes

• fix sep-10 transaction post - clone request header - see #38
• fix revoke sponsorship operation (revokePreAuthTxSigner, revokeSha256HashSigner)
• improve documentation