Enable setting client certificate parent

[torntrousers]

This is similar to #39 in that it enables setting the parent certificate of a client certificate, but the difference to #39 is that it works when the client certificate private key is using an ECC508/608 key slot.

This change is based from #43 and with these two changes it makes it possible to use AWS JITP with an Arduino Nano 33 IoT with the private key stored in the ATECC608A.

[github-actions]

Memory usage change @ 96cba2d

Board	flash	%	RAM for global variables	%
arduino:megaavr:uno2018	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:samd:mkr1000	💚 -96 - 0	-0.04 - 0.0	0 - 0	0.0 - 0.0
arduino:samd:mkrgsm1400	💚 -88 - 0	-0.03 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/SHA1 flash	%	examples/SHA1 RAM for global variables	%	examples/SHA256 flash	%	examples/SHA256 RAM for global variables	%	examples/WiFiSSLClient flash	%	examples/WiFiSSLClient RAM for global variables	%	examples/MKRGSMSSLClient flash	%	examples/MKRGSMSSLClient RAM for global variables	%
arduino:megaavr:uno2018	0	0.0	0	0.0	0	0.0	0	0.0
arduino:samd:mkr1000	0	0.0	0	0.0	0	0.0	0	0.0	-96	-0.04	0	0.0
arduino:samd:mkrgsm1400	0	0.0	0	0.0	0	0.0	0	0.0					-88	-0.03	0	0.0

Click for full report CSV

Board,examples/SHA1<br>flash,%,examples/SHA1<br>RAM for global variables,%,examples/SHA256<br>flash,%,examples/SHA256<br>RAM for global variables,%,examples/WiFiSSLClient<br>flash,%,examples/WiFiSSLClient<br>RAM for global variables,%,examples/MKRGSMSSLClient<br>flash,%,examples/MKRGSMSSLClient<br>RAM for global variables,%
arduino:megaavr:uno2018,0,0.0,0,0.0,0,0.0,0,0.0
arduino:samd:mkr1000,0,0.0,0,0.0,0,0.0,0,0.0,-96,-0.04,0,0.0
arduino:samd:mkrgsm1400,0,0.0,0,0.0,0,0.0,0,0.0,,,,,-88,-0.03,0,0.0

[aentinger]

@ffontaine what do you think?

[ffontaine]

It'm ok with the changes made in this PR.

[aentinger]

LGTM 👍 Thank you @torntrousers 🚀

[torntrousers]

Thats great, thank you. Any chance of a new release?

[aentinger]

I'd like to wait the outcome of #45 and afterwards I'd to a new release.

[aentinger]

@torntrousers here you've got it: https://github.com/arduino-libraries/ArduinoBearSSL/releases/tag/1.7.0

[torntrousers]

thanks!