aiortc: DTLS Bio read buffer is to small and not compatible with ecdsa-with-SHA256 key/cert with CA

With a Janus DTLS certificate with 4096 size, DTLS handshake goes in timeout, the transsaction never succeed.

a related issue was solved few year ago on janus side: https://github.com/meetecho/janus-gateway/issues/252

after investiguation it appears that the ssl bio_read buffer is too small to read the full certificate.

see https://github.com/aiortc/aiortc/blob/d30b6f75d4457204b1ab336106a248d535be053a/src/aiortc/rtcdtlstransport.py#L630

I do some test ad I suggest to increase the bio_read buffer size from:

try:
            data = self.ssl.bio_read(1500)
        except SSL.Error:
            data = b""
...

to a bigger value

try:
            data = self.ssl.bio_read(8192)
        except SSL.Error:
            data = b""
...

It will solves also this closed issue: https://github.com/aiortc/aiortc/issues/346

an alternative but not tested, could be to read in a loop all the bio_read data, and send the full buffer after at https://github.com/aiortc/aiortc/blob/d30b6f75d4457204b1ab336106a248d535be053a/src/aiortc/rtcdtlstransport.py#L634